Diagnosing common infectious diseases – What do all those new tests, titers, PCRs, and blue dots really mean? Part 2: cats (Proceedings)

In some ways, diagnosing disease in cats is more difficult than in dogs. Cats often show vague signs of illness no matter the cause. Diagnostic testing is very important but results are subject to interpretation. The following reviews some tips for identifying feline infectious diseases using immunology.

Viral diseases

Panleukopenia

The same fecal antigen (Ag) tests used for canine parvo can be used for diagnosis. As with dogs, false positive results can occur 3-10 days after vaccination with a modified-live product. False negatives occur when antibody (Ab) binds to the fecal antigen. Hemagglutination inhibition (HI) or virus neutralizing (VN) antibody levels may be measured to indicate vaccine response or exposure.

Herpes

Ab titers are not useful for diagnosis, as they can be high or low independently of clinical signs. Virus detection in the early stages of infection can be done with IFA on swabs/scrapings of affected areas such as the conjunctiva, nasal mucosa, or oral cavity. False negatives are common. Virus isolation takes several days to weeks but has a higher sensitivity. PCR, especially real-time PCR, can be useful although cats with latent infections and "carriers" will test positive even without clinical signs.

Calici

As with herpes, Ab detection is not used unless paired titers (3-4 weeks apart) are saved for retrospective diagnosis. IFA and virus isolation are available, with samples taken from multiple sites. Recent vaccination can result in false positives for both herpes and calici. Real-time PCR testing is offered by some reference labs, often as part of a "feline upper respiratory disease" panel.

Leukemia

FeLV virus (antigen) is detectable by ELISA, IFA, and PCR. Ab titers are not clinically useful. ELISA is the most commonly performed test both in-clinic and at reference labs. A positive result indicates the presence of p27, an internal virus protein. Serum, plasma, whole blood, saliva, and tears may be used but the latter two are less accurate. False positives and false negatives are seen with FeLV ELISA testing. Anti-mouse antibody, if present in the cat due to eating mice, may interfere with the test (false positive). False negatives occur in cases of lab error, poor-quality specimens, or if viremia is too low to detect.

IFA testing for FeLV is performed on smears of whole blood, buffy coat cells, or bone marrow aspirates. Fresh samples without anticoagulant are preferred (use blood directly from syringe to make smears, not from a blood tube). IFA detects p27 within cells, instead of simply circulating in the bloodstream, and a positive result indicates a bone marrow infection and worse prognosis. Bone marrow aspirates are also good samples for IFA testing. PCR assays are not standardized but may detect latent infections in cats testing negative on ELISA and IFA. The clinical significance is unknown. Vaccination does not interfere with testing.

If a healthy cat tests positive on ELISA with a screening test, there is a reasonable chance that the result is a false positive or the infection will be temporary. Some clinicians wait 3-4 weeks and then repeat the ELISA test. Others may submit an IFA test right away. If the IFA is positive, then the cat is almost certainly to be infected and should be isolated from other cats. If IFA-negative, there's still a chance the cat is infected so isolation should continue until a followup ELISA test is negative. Discordant results are difficult to interpret, and clients should be informed that no diagnostic test for FeLV is 100% accurate in every situation.

Immunodeficiency

FIV antigen occurs in very low levels in the bloodstream and is therefore difficult to detect. Instead, Ab testing is used to indicate exposure and infection. The detectable Ab response after exposure takes 1-3 months. ELISA testing is used in-clinic and at reference labs, but both false negatives and false positives are possible. If female cats are infected, their kittens will test positive for up to 4-6 months of age. Adult cats may also be falsely positive if cross-reactions occur (especially if whole blood is used instead of serum or plasma), or if any lab errors. A confirmation test should be run on all ELISA positive samples by Western blot. Both disease and maternal Ab are detected with Western blotting, but this assay has a higher specificity. Vaccination for FIV results in positive Ab tests by either method. PCR testing has been advocated to differentiate vaccinated from infected cats but to date this hasn't been proven (both false positive and false negative results occur with PCR). A discriminant ELISA test has been developed to identify vaccinates from exposed cats, but is not commercially available.

Infectious peritonitis

FIP is more difficult to diagnose than the other feline viral diseases. As FIP is caused by a coronavirus, Ab titers can be measured. However, nonpathogenic enteric coronavirus (which many cats have been exposed to), causes positive Ab results. The mutated pathogenic corona does not have any unique features that allow differentiation from nonpathogenic strains. In the "wet" (effusive) from of FIP, Ab titers can be high, low, or even negative as large amounts of viral Ag can bind the Ab. Cats with the "dry" (noneffusive) form typically have high Ab titers, but a low or negative does not absolutely rule out FIP. Cats in crowded, stressful situations (especially catteries) often have high Ab titers with no disease.

Beware of any lab test that offers a "specific" FIP test (Ab or PCR) or one that interprets results for you. Research is currently looking into the 7b protein which is expressed by some infected cats. To date, clinically affected cats are testing positive to 7b antibody but unfortunately some healthy cats also test positive. In cats with neurologic signs, detection of coronaviral Ab in cerebrospinal fluid (CSF) has been recommended as a diagnostic test. A recent study did not support this finding, and there were many false positives and false negatives. A new PCR test detecting mRNA replicating in macrophages outside the GI tract is available from Auburn University. The sensitivity, specificity, and predictive values have not been reported. Despite much effort by diagnostic labs to come up with an "FIP test", we are still left with histopathology as the gold standard for diagnosis.

Rickettsia and mycoplasma diseases

Ehrlichiosis

Cats are occasionally infected with various species of Ehrlichia. The clinical signs are similar to dogs (fever, lethargy, anorexia, sometimes joint pain, anemia, dyspnea. Positive antibody titers are not consistent, so examination of blood smears for morulae or PCR testing is recommended. Real-time PCR testing for flea- and tick-borne organisms is available at some reference labs.

Anaplasmosis

As in dogs, this is spread by Ixodes sp. ticks and rarely may be seen in outdoor cats in endemic areas. Antibody titers and PCR assist in diagnosis.

Hemobartonellosis

This organism is now identified as Mycoplasma haemofelis. A gradual or sudden onset of anemia is a characteristic finding, along with anorexia, lethargy, icterus, tachypnea, and other signs. The organism is seen on stained blood films about 50% of the time in the acute phase. PCR is another valuable diagnostic tool in sick cats or in screening of healthy cats. Serology is generally not useful.

Bacterial diseases

Bartonellosis

These gram-negative bacteria are found both in healthy cats and those with various clinical signs and diseases. Exposure is common in many parts of the U.S., especially in areas with heavy flea populations. Ticks may also be a vector. In experimental infections, fever, lethargy, anorexia, and enlarged lymph nodes are possible. However, clinical signs are usually mild and transient. Naturally infected cats may also have no signs or very mild signs such as a short-term fever. Research is ongoing to determine if Bartonella exposure is associated with diseases such as gingivostomatitis.

Culture of blood or other tissues is considered the gold standard. However, culture is difficult and time-consuming. PCR documents the presence of DNA but does not prove infection, and both false positives and false negatives are seen. Serologic testing using antibodies against Bartonella species is available using IFA, ELISA, or Western blot. A positive result indicates exposure but not necessarily clinical disease. Despite marketing efforts advocating screening all cats for Bartonella antibodies, the AAFP does not currently make a recommendation due to lack of evidence. Antibiotic treatment of healthy cats simply because of a positive antibody titer is not necessary.

Lyme disease

Cats can be seropositive to Borrelia burgdorferi by ELISA or IFA testing. However, clinical signs or true infection is rare. Arthritis or meningitis may be clinical signs of Lyme disease in cats.

Fungal diseases

Histoplasmosis

Clinical signs are nonspecific and may include depression, fever, anorexia, and sometimes dyspnea. Organisms may be seen on stained specimens from bone marrow, lung, or lymph node aspirates. Serologic testing for antibodies is available but both false negatives and false positives are common. An antigen detection assay is available for this and other fungal diseases (MiraVista Diagnostics).

Cryptococcosis

Upper respiratory disease is common in cats, and ocular/neurologic involvement may be seen. Stained smears of affected tissues are diagnostic or the organism can be cultured. Cryptococcal capsular antigen tests are available that may be performed on serum or CSF. Even a low titer of 1:2 is considered positive.

Coccidiomycosis

Infected cats usually have skin lesions along with fever, anorexia, and weight loss. Bone lesions are less common. Cytology is diagnostic but false negatives are common. Antibodies are detected by either tube precipitin or complement fixation tests where IgM and IgG predominate, respectively. Antigen testing is preferred (see canine information in previous Proceedings).

Protozoal diseases

Toxoplasmosis

The life cycle of Toxoplasma gondii is complex and signs of illness in cats are variable. Most exposed cats are not clinically affected, but tissue cysts persist for life. Approximately 30% of cats in the U.S. have antibodies to Toxoplasma. IFA, ELISA, Western blot, and agglutination methods are used to detect Ab. A positive IgM titer of 1:64 or higher indicates current infection or recent exposure and is considered diagnostic if clinical signs are consistent. However, a positive IgG titer indicates past exposure, as IgG remains elevated for years or a lifetime without clinical illness. Therefore, a single high IgG titer should not be used to diagnose sick cats. Acute and convalescent samples (2-4 weeks apart) usually show a 4-fold or greater increase in either IgM or IgG titers in acute cases.

Giardiasis

Traditionally this parasite was diagnosed on fecal exams, either direct saline smear or zinc sulfate centrifugation flotation. ELISA and IFA tests are now available at reference labs along with an in-clinic ELISA (SNAP Giardia, IDEXX) to detect a protein associated with cyst formation in the intestines. Experience to date suggests that these tests are very accurate in diagnosing Giardia. However, there is some question as to how soon the assays will turn negative after treatment. There are reports of dogs and cats testing positive for weeks or months after appropriate drug therapy and resolution of clinical signs. Some of these cases may be reinfections or the result of a low level of persistent organisms in the GI tract. Screening of healthy animals is also controversial but may be indicated because of zoonotic potential, especially if immunosuppressed humans are in contact.

Cryptosporidiosis

Healthy cats do not often show signs of illness after exposure to cryptosporidia. However, small-bowel diarrhea that may be chronic is seen in some affected cats. Cysts may be seen in direct fecal smears or flotations. IFA testing is available at reference labs and is recommended for workups of kittens and cats with unexplained or chronic diarrhea.

Tritrichomonas

Kittens and young adults with chronic diarrhea should be tested. Culture (pouch testing) is inexpensive and generally accurate. PCR testing is more sensitive and available at some reference labs, often as part of a "feline diarrhea" panel.